4,6-Diamino-resorcinol dihydrochloride is an important building block for plastics, in particular for polybenzoxazoles (Macromolecules 1981, 915).
EP 0,402,688 describes the synthesis of 4,6-diamino-resorcinol from 1,3-dichloro-4,6-dinitrobenzene by reaction with sodium benzylate to give 1,3-dibenzyloxy-4,6-dinitrobenzene and subsequent catalytic hydrogenation. A disadvantage of the process described is the indicated way of carrying out the heterogeneous catalytic hydrogenation of 1,3-dibenzyloxy-4,6-dinitrobenzene on a Pd/activated carbon catalyst as a batch hydrogenation which, in the form described, does not permit economical recycling of the expensive noble metal catalyst. A further disadvantage is the unsatisfactory product quality, caused by the way, as described, of carrying out the heterogeneous batch hydrogenation, and the low stability of 4,6-diamino-resorcinol in the free form, in particular in water-containing solvents. A further disadvantage is the low solubility of free 4,6-diamino-resorcinol in organic solvents, which leads to an uneconomically large dilution during the operation of the catalytic hydrogenation and the isolation of the product.
It has now been found that 4,6-diamino-resorcinol dihydrochloride can be prepared from 1,3-dibenzyloxy-1,4-dinitrobenzene by reacting 1,3-dibenzyloxy-4,6-dinitrobenzene on a noble metal catalyst in a two-phase mixture of dilute aqueous hydrochloric acid and an organic solvent, which is immiscible or miscible only to a limited extent with dilute aqueous hydrochloric acid, at optionally elevated pressure and normal or elevated temperature by a catalytic hydrogenation, preferably a catalytic pump hydrogenation, to give 4,6-diamino-resorcinol dihydrochloride.